What is the circadian rhythm and how does it control your body?

The answer

The circadian rhythm is an internal biological clock driven by a master pacemaker called the suprachiasmatic nucleus (SCN), a cluster of about 20,000 neurons in the hypothalamus directly above the optic chiasm. The SCN maintains an approximately 24-hour cycle through transcription-translation feedback loops of clock genes (CLOCK, BMAL1, PER, CRY) that oscillate in every cell of the body. Light detected by specialized melanopsin-containing retinal ganglion cells resets the SCN daily, synchronizing internal time with the external light-dark cycle. The circadian system regulates sleep-wake cycles, hormone release (cortisol peaks in the morning, melatonin at night), body temperature, metabolism, immune function, and even gene expression. Chronic disruption of circadian rhythms, as in shift workers, is associated with increased risks of cancer, obesity, diabetes, and cardiovascular disease.